Metal film resistor



United States Patent Q Patented Oct. 7, 1958 METAL FILM RESISTOR Melvin Tierman, North Adams, Mass., assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts No Drawing. Application June 16, 1953 Serial No. 362,164

3 Claims. (Cl. 20173) The present invention relates to a new and improved variety of metal film resistor.

It is common knowledge at the present time that various thin metal layers can be satisfactorily used as resistance elements. A great deal of Work has been done in attempting to determine the optimum type of metallic composition for use with such resistors. Unfortunately, this work has not been extremely successful to date; and prior to the present invention, it is not believed that anyone produced a metal film resistor possessing as low a temperature coetficient of resistance over Wide ranges of resistance values as desired for many purposes. Further, it is not believed that prior to the present invention that anyone had produced metal film resistors exhibiting extreme stability to load life tests.

It is an aim of the present invention to produce new and improved metal film resistors having very low temperature coetficients of resistance. Further, it is an object of the invention to produce resistors which are highly stable over long load life tests, and which can be easily and conveniently made without the high degree of control necessary in forming the prior art related units. Further objects of the invention, as well as the advantages of it, will be apparent from the body of this specification and the appended claims.

The basic feature of the instant invention rests upon the discovery that the addition of approximately 2 /2% of rhodium to gold-palladium resistance compositions containing from 40 to 70 parts of gold and 30 to 60 parts of palladium greatly improved the characteristics of resistance films of these alloys so as to achieve the objects indicated above.

Surprisingly enough, no equivalents are known for the rhodium used for the purpose of stabilizing a gold-palladium resistance film composition as indicated above. While the figure 2 /z% is indicated above, it is to be understood that from 1 /2% to about 3 /2% of rhodium can be employed without departing from the scope of the instant invention.

To illustrate the function of rhodium within gold-palladium resistance films, it should be mentioned that the new resistance films are subject to a change of less than 0.2% in resistance value over a 500 hour life test, wheresilver end caps used with gold-palladium film resistors tended to effect a change in the composition of the alloy film adjacent to these caps. For some unknown reason the small proportion of rhodium added in accordance with this invention prevents this diffusion. Also, the new composition films of the invention are less sensitive to changes in manufacturing conditions than the prior art film resistors.

The procedure used in forming new film resistors described herein is substantially that known to the prior art. Briefly it involves making up a resinate solution containing the metals used in the final film in the proportion in which they are used in this film, spraying this resinate onto an inert core, such as a steatite core, and drying the deposited resinate as by the use of infra-red lamps. This coating cycle is repeated until the desired thickness of the resinate is built-up upon the inert base. Then this coated base is dried at a temperature in excess of the boiling point of the solvent, but below about 250 C., for a short period until all traces of the solvent are gone. Finally, the unit is fired at a temperature of from about 320 to about 400 F. for a period of from 5 to 30 minutes. Appropriate terminals are attached using known procedures. If desired, metal films as produced above can be cut into small thin bands as by the use of a lathe or a milling machine. 1

It is to be realized that the unusual effect of the addition of rhodium to the alloy useful as thin metallic resistance films is not solely limited to three component alloy systems, but may be extended to alloy systems having four or more metallic components. As an example of this, the addition of a minor amount of rhodium to an alloy having a major amount of gold and palladium with a further minor amount of platinum results in a four-component system which is characterized by the formation of thin metallic films of large resistivity per unit volume, low chemical reactivity, much improved temperature coefiicients of resistance over the prior art cases, and exceptional operational stability over long periods of use.

As an example of the hereinbefore described invention, a ceramic core was coated by spraying with six successive coats of metal organo resinate solution containing 10 /2 parts of palladium, 15.8 parts of gold, and .7 part of rhodium. After the final coating Was placed on the ceramic rod the unit was fired to decompose the organo resinate composition and leave the surface film of the metal alloy. Silver paint was placed on the opposed ends of the metal film and was fired thereafter to fuse the silver so that low resistance contacts were made with the metal film. On ten units made in this manner the initial resistance of the films was found to vary from 600 to 900 ohms and thereafter each individual unit was spiralled to 100,000 ohms resistance. The units were placed on a 1 watt load test with an ambient temperature of 40 C. being maintained and after 750 hours had an average change of less than percent with none of the units in excess of this figure.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof, except as defined in the appended claims.

What is claimed is:

1. An electrical resistance having an inert electrically non-conductive base with a fired-on thin film of an alloy consisting of essentially of from about 40% to about by weight of gold, from about 30% to about 60% by 3 4 Weight of palladium and from about 1.5% to about 3.5% References Cited in the file of this patent by weight ofrhodium and having a temperature co'efii- UNITED STATES PATENTS cient of resistivity of the order of 0.025% per C.

2. An electrical resistor comprising the electrical re- 1,169,753 Paschko 1916 sistance element of claim 1 With metal terminal connec- 5 1,339,505 Fahrenwald May 1920 tions attached to spaced portions of said film, the metal 1,789,733 132611521161 1931 of said terminal connections being substanially unditfused 2,440,691 hra May 1948 into said electrical resistance element.

3. The combination of claim 2 in which the terminal FOREPGITI PATENTS connections include fired-on silver coatings on the alloy 10 1 ,025 Great Brltaln Feb. 19, 1917 film. 585,545 Germany Oct. 21, 1930 

1. AN ELECTRICAL RESISTANCE HAVING AN INERT ELECTRICALLY NON-CONDUCTIVE BASE WITH A FIRED-ON THIN FILM OF AN ALLOY CONSISTING OF ESSENTIALLY OF FROM ABOUT 40% TO ABOUT 70% BY WEIGHT OF GOLD, FROM ABOUT 30% TO ABOUT 60% BY WEIGHT OF PALLADIUM AND FROM ABOUT 1.5% TO ABOUT 3.5% BY WEIGHT OF RHODIUM AND HAVING A TEMPERATURE COEFFICIENT OF RESTIVITY OF THE ORDER OF 0.025% PER*C. 